This invention relates to p-hydroxymethylbenzoic acid. More particularly, it relates to a process for the preparation of p-hydroxymethylbenzoic acid with a low level of 4-carboxybenzaldehyde. Even more particularly, the invention relates to a process for preparation of p-hydroxymethylbenzoic acid (p-HMBA) by electrochemical reduction of terephthalic acid, to operation of the electrolysis cell by treatment of the cathode to obtain a mercuric overlay as an amalgam on the cathode surface, the addition of a soluble salt of mercury, which regenerates the amalgam overlay, to the electrolysis catholyte solvent, the reduction of the terephthalic acid being performed at a voltage of at least 6 volts with current efficiency being maintained at a level of at least 80%, the reduction of the 4-carboxybenzaldehyde being performed at a voltage of not greater than 4 volts. The process, as a continuous process, is obtained by use of two electrolysis cells in series, wherein p-hydroxymethylbenzoic acid (p-HMBA) containing by-product impurities from one electrolysis cell is transported to a second electrolysis cell for reduction of by-product impurities. In a batch operation, the entire process is obtained in one electrolysis cell.
Numerous methods are known for the preparation of p-hydroxymethylbenzoic acid. Among other methods, some of these are based on the saponification of a corresponding halogenmethyl compound, such as p-chloromethylbenzoic acid or the esters thereof or p-chloromethylbenzonitrile. For example, several methods for the synthesis of p-hydroxymethylbenzoic acid are taught in U.S. Pat. No. 4,130,719. The electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid is taught in commonly-assigned application Ser. No. 319,120, filed Nov. 9, 1981, now U.S. Pat. No. 4,381,229.
p-Hydroxymethylbenzoic acid must be free from by-products when it is to be employed in polycondensation reactions, such as in the preparation of polyesters. However, most of the known processes for the preparation of p-hydroxymethylbenzoic acid do not yield the acid free from by-products. Thus, for example, during the saponification of highly pure p-chloromethylbenzoic acid in a faintly alkaline aqueous medium, up to 10% of dibenzylether-4,4'-dicarboxylic acid is always produced.
In other methods, as for example, in the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid many competing reactions take place in the electrolysis cell. The resulting presence of 4-carboxybenzaldehyde (4-CBA), dihydroxymethylbenzene, toluic acid and other impurities render the resulting p-hydroxymethylbenzoic acid undesirable for use as a monomer for polymer applications without further expensive purification.
It is well-known that in the cathodic reduction of carboxylic acids that two types of products can result, either the corresponding aldehyde in a two-electron process or the hydroxymethyl compound in a four-electron process where the aldehyde is further reduced to the alcohol. (M. Baizer, Organic Electrochemistry, Dekker, N.Y. (1973), 414) The alcohol can be further reduced to the methyl group.
Accordingly, in the preparation of p-hydroxymethylbenzoic acid to obtain the crude acid, many by-products can also be produced, among which are 4-carboxybenzaldehyde, dihydroxymethylbenzene and toluic acid. 4-Carboxybenzaldehyde is a particularly undesirable impurity because it acts as a chain-stopper stopper during polyesterification. Although 4-carboxybenzaldehyde is difficult to remove by physical means, it can be hydogenated to toluic acisd and other derivatives. Toluic acid also acts as a chain-stopper during polyesterification. Toluic acid can be efficiently removed by cooling and crystallizing crude p-hydroxymethylbenzoic acids containing it. 4-Carboxybenzaldehyde also can be hydrogenated to the hydroxymethyl compound, i.e., p-hydroxymethylbenzoic acid, in an electrochemical process as is taught by Baizer, mentioned above, but hydrogenation of terephthalic acid in an electrochemical process results in increased quantities of 4-carboxybenzaldehyde despite the concurrent hydrogenation of 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid. Accordingly, under the usual conditions used to obtain electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid, the presence of the resulting by-products in the product stream renders the resulting p-hydroxymethylbenzoic acid unfit for polyesterification without further extensive purification.
Accordingly, it is an object of the present invention to develop an electrochemical process for the manufacture of p-hydroxymethylbenzoic acid from terephthalic acid which avoids the aove disadvantage of producing more 4-carboxybenzaldehyde (4-CBA) than is hydrogenated to p-hydroxymethylbenzoic acid, and thus the level of the 4-CBA and other impurities is increased with continued operation. It is an object of the present invention to provide a process for producing of p-hydroxymethylbenzoic acid wherein production of 4-carboxybenzaldehyde, dihydroxymethylbenzene and toluic acid is minimized. It is a further object of this invention to provide a process for electrochemical manufacture of p-hydroxymethylbenzoic acid from terephthalic acid wherein the level of 4-carboxybenzaldehyde in the resulting product is equal to or less than 500 parts per million (ppm). It is a further object of this invention to obtain high current efficiency in the reduction of terephthalic acid to p-hydroxymethylbenzoic acid and to obtain a low level of by-product production, specifically 4-carboxybenzaldehyde, despite high current efficiency obtained in the reduction of terephthalic acid which, in turn, produces a high level of impurities, such as 4-carboxybenzaldehyde, dihydroxybenzene and toluic acid. Other objects will appear hereinafter.